1. Field of the Invention
This invention relates generally to nuclear reactor fuel element support grids and more particularly to support grids that include diagonal retaining springs.
2. Background Information
Fuel assemblies for nuclear reactors are generally formed from an array of elongated fuel elements or rods maintained in a laterally spaced relationship by a skeletal support structure, including a plurality of longitudinally spaced support grids, a lower end fitting, and an upper end fitting. The fuel assembly skeleton also includes guide tubes and instrumentation thimbles, which are elongated tubular members symmetrically interspersed among and positioned coextensive with the fuel element locations. The guide tubes and instrumentation thimbles are fixedly connected to the support grids to provide the structural coupling between the other skeletal members. The support grids each define an array of fuel rod support compartments or cells and have a perimeter that is configured in one of a number of alternate geometrical shapes that is dictated by the reactor core design. Nuclear fuel grids for commercial pressurized water reactors employing square fuel assemblies can typically have between 14 and 17 cells per side. Other polygonal array designs are also employed, such as the hexagonal array illustrated in U.S. Pat. No. 5,303,276, issued Apr. 12, 1994.
One typical fuel element support grid design includes a generally polygonal perimeter surrounding an interior lattice array. A plurality of fuel element compartments or cells within the perimeter are defined by a number of evenly spaced, slotted, interlocked lattice forming members or straps, which are welded to the perimeter and joined to each other by small nugget welds at the ends of their lines of interception along the slotted locations.
Each interior lattice forming member is slotted over one half of its width along its lines of intersection with the other grid forming members of the array. The members are assembled and interlocked at the lines of intersection with the slot in one member fitting into the opposing slot in the crossing member in an "egg-crate" fashion. This egg-crate design provides a good strength to weight ratio without severely impeding the flow of coolant that passes through the grid in an operating nuclear reactor. The lattice-forming members typically include projecting springs and dimples for engaging and supporting the fuel elements within some of the grid compartments. The springs provide axial, lateral and rotational restraint against fuel rod motion during reactor operation under the force of coolant flow, during seismic disturbances, or in the event of external impact. These spacer grids also act as lateral guides during insertion and withdrawal of the fuel assemblies from the reactor.
One of the operating limitations on current reactors is established by the onset of film boiling on the surfaces of the fuel elements. The phenomenon is commonly referred to as departure from nuclear boiling (DNB) and is affected by the fuel element spacing, system pressure, heat flux, coolant enthalpy and coolant velocity. When DNB is experienced, there is a rapid rise in temperature of the fuel element due to the reduced heat transfer that occurs under these conditions as a result of the gaseous film that forms on portions of the fuel element surface, which can ultimately result in failure of the fuel element if it was to continue. Therefore, in order to maintain a factor of safety, nuclear reactors must be operated at a heat flux level somewhat lower than that at which DNB occurs. This margin is commonly referred to as the "thermal margin."
Nuclear reactors normally have regions in the core that have a higher neutron flux and power density than other regions. The variation in flux and power density can be caused by a number of factors, one of which is the presence of control rod channels in the core. When the control rods are withdrawn, these channels are filled with coolant, a moderator, which increases the local moderating capacity and thereby increases the power generated in the adjoining fuel. In these regions of high power density known as hot channels, there is a higher rate of enthalpy rise than in other channels. These hot channels set the maximum operating conditions for the reactor and limit the amount of power that can be generated, since it is in these channels that the critical thermal margin is first reached.
The prior art has attempted to reduce the variation in power density across the core and thus increase the DNB performance by providing coolant flow deflector vanes as an integral part of the fuel support grids. The vanes improve performance by increasing heat transfer between the fuel rods and the coolant downstream of the vane locations. The vanes are especially beneficial in the regions adjoining the hot channels, which are the fuel element positions adjacent to the control rod guide tube locations.
To take full benefit of the vanes, it is also desirable to streamline the remaining grid components, i.e., the lattice straps, including the springs, dimples and welds, to reduce the turbulence generated upstream from the vanes. Other objectives in optimizing fuel grid designs include minimizing grid pressure drop and maximizing grid load carrying strength. The springs, which provide the force for holding the fuel rods in position, are normally formed from cut-out sections of the lattice forming members that protrude into the fuel rod support compartments. The spring force applied is designed as a balance between the forces necessary to provide the axial, lateral and rotational restraint required to hold the fuel elements in position and that which will score or otherwise damage the surface of the fuel element as it is threaded into the assembly during manufacture. To both avoid damage to the fuel element and provide maximum restraining forces it is desirable to maximize the contact area that the spring has with the fuel rod as well as the flexure of the spring. A preferred method for achieving maximum contact area is to provide a diagonal spring which extends from a lower portion of one of the walls of the fuel support compartment to a diagonally opposed upper portion of the same wall as illustrated in the prior art design illustrated in FIG. 2. FIG. 2 shows a single wall section of a grid lattice strap 110 with a diagonal spring 112. The cut-out sections 114 protrudes into an adjacent fuel element support compartment and forms the dimple for providing point contact support for an adjacent fuel rod which is pressed against the dimple by a similarly formed spring extending inwardly from the opposite wall of that adjacent compartment. Typically a fuel rod support compartment is provided with springs on at least two walls and dimples on the opposing walls to center the fuel rods and provide maximum coolant flow around their surface. The prior art also provided cut-out sections 116, shown in FIG. 2, to reduce the mass of wall material around the spring and thus increase it flexibility.
As shown in patent application Ser. No. 08/887,017 (Docket ARF96-003 it is desirable to locate the mixing vanes 120 over the grid compartments that support the fuel element, to enhance heat transfer. It has been found however, that the vanes increase the pressure drop in the fuel support compartments. That creates a pressure differential between the fuel support compartments that adjoin the control rod guide tube and instrument thimble locations and the tube and thimble locations, that do not have mixing vanes. As a result, during operation the coolant flowing through the grid compartments adjoining the guide tube and thimble locations tends to seek the path of least resistance and flow out the windows 116 on either side of the diagonal 112 spring and up through the thimble and guide tube locations. The result is reduced heat transfer in the area that most needs it and less efficient use of the vanes.
Accordingly, an improved grid structure is desired that improves DNB performance. It is an object of this invention to achieve that result by minimizing the leakage path around the grid springs, while maintaining the spring's flexibility. It is a further object of this invention to improve upon the flexibility of the diagonal spring design without reducing the crushable strength of the grid.